Problem: $z=-2-5i$ Find the angle $\theta$ (in radians ) that $z$ makes in the complex plane. Round your answer, if necessary, to the nearest thousandth. Express $\theta$ between $-\pi$ and $\pi$. $\theta=$
The strategy We can find the angle $\theta$ of any complex number $z$ by solving the following equation. $\tan\theta=\dfrac{\text{Im}(z)}{\text{Re}(z)}$ This equation usually has two solutions in the interval $[-\pi,\pi]$. We can find the appropriate solution by reasoning about the quadrant in which $z$ lies. Solving for $\theta$ $\begin{aligned}\tan\theta &= \dfrac{\text{Im}(z)}{\text{Re}(z)}\\\\ \tan\theta&=\dfrac{-5}{-2}\\\\ \theta&=\arctan\left(\dfrac{5}{2}\right)&\text{Take the arctangent of both sides}\\\\ \theta&\approx1.190\end{aligned}$ Using the identity $\tan(-\pi+\theta)=\tan(\theta)$, we know that the following is also a solution of the equation. $-\pi+(1.190)=-1.951$ In order to determine which of these two solutions is the angle of $z$, let's take a look at its graphical representation. ${3}$ ${6}$ ${\llap{-}6}$ ${3}$ ${6}$ ${\llap{-}6}$ $Im$ $Re$ $z$ $\theta$ $Re(z)$ $Im(z)$ Since $z$ lies in Quadrant $\text{III}$, its angle must be in the interval $(-\pi,-\dfrac{\pi}{2})$. Therefore, $\theta=-1.951$. Summary $\theta=-1.951$